Method for producing a connection between composite parts

ABSTRACT

A method for joining together constructional parts which are made of a composite material and extend transversely to one another, such as a flange and a web of a profile, includes the steps of:
         placing the constructional parts in the desired position with respect to one another so as to form a connecting region in which one of the constructional parts abuts against or in proximity to the other constructional part,   causing a thermoplastic material to flow out into the connecting region,   fusing together the constructional parts under the influence of heat and pressure.

The invention relates to a method for joining together parts made of a composite material. Parts of this type consist of one or more layers of fibers embedded in a matrix made of plastics materials. The plastics material matrix may be a thermoplastic or a thermoset; the choice of the plastics material matrix is usually determined by the properties which the finished product is required to have.

In a conventional manner of joining together two parts made of composite material, the parts are placed flat against one another. Subsequently, the thermoplastic matrix is melted under the influence of heat, after which the parts are fastened to one another under pressure by local fusion. In such a manner of fastening, a flange is usually formed on one or both parts, at the location of which flange or flanges the connection is then produced. Examples include the joining of a web plate provided with flanges to two accompanying purlins so as to form an I-shaped beam.

However, producing a connection between two composite parts in this way is not without its drawbacks. Firstly, the presence of one or more flanges on one of the composite parts, intended to produce the connection, can be problematic. Although a flange shaped in this way is readily possible in the case of a composite part which is otherwise flat, in the case of non-flat composite parts, such as an undulatory web for an I-shaped beam, shaping a flange is awkward, since this would also impart a non-flat shape to the bend line. The attachment of a flange by bending with respect to a non-straight bend line is, however, rendered difficult as a result of the fact that the deformation associated therewith causes high stresses in the plane of the flange and the web. A further drawback is that the flanges in the finished construction do not always contribute efficiently to the overall strength and rigidity properties. Although they do ensure the necessary connection between the web and purlins, this does not always lead to an effective contribution in the aforementioned properties.

In certain cases, the additional weight associated with flange connections of this type can be a drawback. This is relevant, in particular, in applications in aviation and space travel. Often the flanges are formed by extending the layers of fiber material continuously into the flange or flanges. If there is a specific prescribed minimum thickness of the flanges, the remainder of the respective composite part must then also have a specific layer thickness, and this is not always necessary for reasons of rigidity and strength.

The object of the invention is therefore to provide a method for joining together composite parts that does not have these drawbacks. That object is achieved by means of a method for joining together constructional parts which are made of a composite material and extend transversely to one another, such as a flange and a web of a profile, including the steps of:

-   -   placing the constructional parts in the desired position with         respect to one another so as to form a connecting region in         which one of the constructional parts abuts against or in         proximity to the other constructional part,     -   causing a thermoplastic material to flow out into the connecting         region,     -   fusing together the constructional parts under the influence of         heat and pressure.

According to the invention, the use of connecting flanges is dispensed with when producing a connection between two composite parts. Instead, a thermoplastic material, which after melting under the influence of heat and pressure forms the complete connection, is allowed to flow out between the abutting edge of one of the composite parts and the directly opposing face of the other composite part. This is possible because the properties of the connection are dictated by the thermoplastic without the layers of fibers proving to be the decisive factor in this regard. This finding therefore allows the production, using less material, namely without the hitherto conventional flanges, of a connection which meets the requirements placed on strength and rigidity.

The thermoplastic material which is allowed to flow out can, for example, derive from the parts to be joined together themselves. Under the influence of heat, the thermoplastic material of the parts becomes fluid in such a way that said thermoplastic material flows out under pressure into the connecting region. Since the amount obtained in this way of thermoplastic material in the connecting region is limited, this variation of the method is particularly suitable for constructions which are subject to relatively light loads. In addition, or alternatively thereto, additional thermoplastic material can be added to the connecting region. The connection is then produced using a relatively large amount of thermoplastic material, so connections produced in this way using additional thermoplastic material are suitable for constructions which are subject to heavier loads.

The connections obtained in this way must be able to meet various requirements. A distinction must be drawn in this regard between the requirement of strength and the requirement of rigidity. The first requirement is critical if the connection must be able to withstand maximum loads. In the case of a conventional connection, the shearing force from the web plate is transferred over a broad width, as determined by the width of the flange or flanges, onto the surface of the purlin (in the case of an I-shaped beam or T-shaped beam). In the case of a product produced using the present method, this width is considerably lower and is limited to the width of the bead made of thermoplastic material.

The method according to the invention may, in particular, include the steps of:

-   -   forming a bead from thermoplastic material,     -   causing the width of the bead to increase, starting from the         abutting leading edge of one constructional part toward the         opposing surface of the other constructional part.

A method of this type is particularly suitable for stiffeners which are attached to floor joists and for annular stiffeners on web plates.

For constructional parts which are subject to relatively high loads, the leading edge of one constructional part can also be widened. This increases the boundary surface area between the web plate and the purlin. The advantage of this is that, for otherwise equal shearing forces, the shear stress in the boundary surface can remain lower.

In particular, this can involve carrying out the steps of:

-   -   splitting the leading edge of one constructional part so as to         form two legs enclosing a longitudinal cavity,     -   filling the longitudinal cavity with thermoplastic material.

The thermoplastic material in the longitudinal cavity is preferably filled with fibers, for example to an amount of 60%. This improves the stability of the filling material in the longitudinal cavity, yielding a stabler construction.

The stability can further be improved by the step of:

-   -   causing the width of the bead to increase from approximately the         width of the leading edge of one constructional part to at most         five times said width on the surface of the other constructional         part. In particular, the width of the bead can be caused to         increase to at most three times the width of the leading edge of         one constructional part.

Another possible embodiment of the method according to the invention includes the steps of:

-   -   placing the leading edge of one constructional part against the         other constructional part,     -   attaching thermoplastic material to the outside of both         constructional parts in the connecting region.

In this method, there is therefore no need to remove the leading edge of the respective constructional part. The original leading edge is placed directly against or almost against the other constructional part. The beads of thermoplastic material on either side of the leading edge ensure effective transmission of force in this embodiment.

The thermoplastic material used in the connecting region is preferably PEI, PEKK or PEEK.

A fiber-reinforced composite material can be used in the method according to the invention. Various materials can be used as the fiber reinforcement; examples include metal-reinforced composite material, or composite material impregnated with PEI, PEKK or PEEK.

The method according to the invention can be carried out in various ways. According to a preferable option, the method according to the invention includes the steps of:

-   -   placing the constructional parts into a mould,     -   causing the thermoplastic material to melt in the mould under         pressure and heat.

As stated hereinbefore, in the case of specific constructions which are subject to relatively heavy loads, the connection between the constructional parts can be obtained by adding thermoplastic material. This can be achieved, in particular, by introducing that thermoplastic material in the form of a filling element. The filling element can be preproduced, for example by means of extrusion or pressing, and be positioned at the desired location in the connecting region. This can involve, for example, applying two filling elements on either side of an abutting constructional part. According to still another option, one or more filling elements can be applied on either side of the abutting constructional part and between that abutting constructional part and the other constructional part. The filling element can consist of a thermoplastic material which is filled with relatively short reinforcing fibers.

To ensure a good connection, use is made in the connecting region of a thermoplastic material which is similar to the thermoplastic material of the composite material of the constructional parts. The method according to the invention also includes the fusing under the influence of heat and pressure of the constructional parts in their entirety.

The mould which is used in this case follows closely the contours of the products to be produced. The overall assembly consisting of the mould and the workpiece, which is enclosed therein and comprises constructional parts consisting of composite and, if appropriate, filling elements, is placed into an autoclave. In the autoclave the assembly as a whole is exposed to heat and excess pressure. The thermoplastic material of the composite parts and of any filling elements melts in this case completely, thus totally fusing said components.

The invention further relates to a construction produced using the method as described hereinbefore. This construction comprises at least two constructional parts which are directed transversely to one another in such a way that the leading edge of one of the constructional parts abuts against or in proximity to a surface of the other constructional part so as to form a connecting region, and also a connection which is made of thermoplastic material which is located in that connecting region. This may be a T-shaped profile, an I-shaped profile and the like.

The invention will be described hereinafter in greater detail with reference to a few exemplary embodiments illustrated in the figures, in which:

FIG. 1 is a side view of an I-shaped beam according to the invention;

FIG. 2 is a cross section taken along the line II-II from FIG. 1;

FIG. 3 shows a variation of the leading-end connection between the constructional parts;

FIGS. 4-8 show further variations;

FIG. 9 shows the mould pertaining to the variation from FIG. 4;

FIG. 10 shows the mould pertaining to the variation from FIG. 8;

FIG. 11 shows the mould pertaining to the detail from FIG. 7 b; and

FIG. 12 shows the mould pertaining to the shell plate from FIG. 7 a.

The I-shaped beam shown in FIGS. 1 and 2 according to the invention consists of a web plate 1, an upper purlin 2 and a lower purlin 3. These purlins 2, 3 are joined together by means of the weld beads 4. In the exemplary embodiment shown, these weld beads 4 extend from the leading edge 5 of the web plate 1 up to the surface 6 of the purlins 2, 3 that faces said leading edge. In the connecting region 10 thus determined, the width of the weld beads increases from the thickness of the web plate 1 to three to five times said thickness at the location of the surface 6 of the purlins 2, 3. This provides good attachment, with relatively low shear stresses, to the purlins 2, 3. This variation is typical of that embodiment of the method according to the invention in which a relatively small amount of thermoplastic material is obtained in the weld bead as a result of thermoplastic material flowing out of the web plate and purlins. In view of the narrow connecting regions thus obtained, a construction of this type is particularly suitable for relatively light loads.

This also applies to the variation from FIG. 3. In this case, the leading edge 5 of the web plate 1 has been removed so as to form two legs 7 enclosing a longitudinal cavity 8. This longitudinal cavity is filled with outflowing thermoplastic material. A relatively large attachment surface area between the leading edge 5 of the web plate 1 and the surface 6 of the purlin 2 can in this way too be obtained in the connecting region 10. The unobstructed continuation of the web plate 1, via the split legs 7 thereof, up to the purlin 2 ensures that a strong, rigid connection is obtained. The low shearing stresses, resulting from the wide nature of the leading edge 5, at the location of the attachment to the purlin also prevent excessive loading of the thermoplastic material.

The variation from FIG. 4 shows an I-shaped beam from which the leading edge 5 of the web plate 1 has not been widened. The web plate 1 extends right up to the purlins 2 in such a way that the leading edges 5 thereof rest almost or entirely against the surface 6 of the purlins 2. Weld beads 9, which are obtained by adding additional thermoplastic material (see below and FIG. 9), are applied on either side of the connecting region 10 thus formed. This provides weld beads 9 having a relatively large cross section, so a connection of this type is particularly suitable for constructions subject to relatively heavy loads.

FIG. 5 shows a connection corresponding to that of FIG. 4, although in this case no additional thermoplastic material has been added. As a result, the weld beads 11 are considerably smaller than that according to the variation from FIG. 4. After all, the weld beads 11 are obtained as a result of the outflow of the thermoplastic material of the purlin 1 and the flange 2 itself, so this construction is particularly suitable for relatively light loads.

The variation from FIGS. 6 a and 6 b relates to a joist, the web plate 1 of which has openings 13 which are reinforced by means of cylindrical flanges 12. The cylindrical flanges 12 are fastened to the web plate 1 by means of the weld beads 11 which can also be obtained by mere outflow of the thermoplastic material of the web plate 1 and of the cylindrical flange 12. In addition, additional thermoplastic material can also be added so as to obtain a thicker weld bead and thus a connection which is able to withstand heavier loads.

The variation from FIGS. 7 a and 7 b relates to a rigidified panel having a shell plate 14 to which stiffeners 15 are fastened. The weld beads 9 are obtained by adding additional thermoplastic material (see below and FIGS. 11 and 12) because a shell plate 14 of this type may be a constructional component which is subject to relatively high loads.

FIG. 8 shows a variation of an I-shaped beam, the flanges 2, 3 of which are fastened in a manner different to the variation from FIG. 4. Between the leading edges 5 on the upper side and underside of the web plate 1 and the top and bottom purlins 2, 3 there are now spaces which are filled with thermoplastic material 23. This thermoplastic material 23 forms a single entity with the weld beads 9 located on either side of the leading edge.

FIG. 9 illustrates the manner in which the I-shaped beam according to FIG. 4 is produced with the aid of the mould 25. The mould 25 consists of the top and bottom mould plates 16, 17 and also the left-hand and right-hand mould pieces 19, 18. These mould plates and mould pieces enclose a forming cavity 24 which has the contour of the final I-shaped beam according to FIG. 4. As is shown, the forming cavity 24 receives the lower purlin 3, the web plate 1 and the upper purlin 2. Filling elements 20, which are extruded or pressed in such a way that they closely follow the contour of the mould pieces 19, 18 and of the purlins 2, 3 and web plate 1, are also received at the location of the corners between the purlins 2, 3 and the web plate 1.

The overall assembly consisting of the mould 25, the purlins 2, 3, the web plate 1 and the filling elements 20 is placed into an autoclave and subjected to heat and pressure. This causes the thermoplastic material of the purlins 2, 3, the web plate 1 and of the filling elements 20 to melt. The thermoplastic material of said components blends in such a way that after cooling the finished I-shaped beam according to FIG. 4 is obtained.

FIG. 10 also illustrates the mould 25, although as used in the production of the I-shaped beam according to FIG. 8. The course of events in the production of this I-shaped beam corresponds broadly to that of FIGS. 4 and 9. In this case, filling elements 21, consisting of the filling elements 20 as used in the variation from FIG. 8 and also the bridges 22, are however attached at the location of the top and bottom leading edges 5 of the web plate 1. These filling elements 20 and bridges 22 form both at the top and at the bottom a single entity which can be obtained, for example, by means of extruding or pressing. The top and bottom leading edges 5 of the web plate 1 are thus set apart from the opposing surfaces 6 of the top and bottom purlins 2, 3. The I-shaped beam according to FIG. 8 is thus obtained when the mould 25 is subjected to excess pressure and heated, as shown in FIG. 10.

FIGS. 11 and 12 show the mould 26 comprising the forming cavity 27 which is used for producing the rigidified panel according to FIGS. 7 a, b. This mould 26 consists of the top mould plate 28 and the bottom mould plate 29 and also of the mould pieces 30. The shell plate 14 and also a stiffener 15 comprising filling elements 20 is received between each two mould pieces 30, the mould plate 28 and the mould plate 29. The rigidified panel according to FIGS. 7 a, b is thus obtained under the influence of heat and pressure, as supplied in an autoclave.

The above-described connections can be used in all types of constructions and in particular in girders for use in aviation and in space travel. Examples of applications of this type include the supporting beams as used in the floor of a cockpit.

LIST OF REFERENCE NUMERALS

-   1. Web plate -   2. Purlin -   3. Purlin -   4. Weld bead -   5. Leading edge of the web plate -   6. Surface of the purlin -   7. Leg of the web plate -   8. Longitudinal cavity in the web plate -   9. Weld bead -   10. Connecting region -   11. Weld bead -   12. Cylindrical flange -   13. Opening in the web plate -   14. Shell plate -   15. Stiffener -   16., 17. Mould plate -   18., 19. Mould piece -   20. Filling element -   21. Filling element -   22. Bridge -   23. Thermoplastic material filling -   24., 27. Forming cavity -   25., 26. Mould -   28., 29. Mould plate -   30. Mould piece 

1. A method for joining together constructional parts, which parts are made of a composite material and which parts extend transversely to one another, the method comprising steps of: within a mould, placing i) a first constructional part adjacent and extending transversely to a second constructional part, and ii) a third constructional part adjacent and extending transversely to the second constructional part, wherein the mould with the first, second, and third constructional parts placed therein define a cavity, the cavity has a contour of a final product to be formed, and each of the constructional parts is a composite material comprising fibers and thermoplastic material; subjecting the mould and the first, second, and third constructional parts therein to heat and pressure sufficient to cause i) the thermoplastic material of the first, second, and third constructional parts to melt completely and become fluid with the thermoplastic material flowing out, under pressure, into the cavity including a first connection region between the first and second constructional parts and a second connecting region between the second and third constructional parts, and ii) fusing of the first, second, and third constructional parts in their entirety; and cooling the fused first, second, and third constructional parts to obtain the final product having an outside contour matching the contour of the cavity of the mould.
 2. A method for joining together constructional parts, which parts are made of a composite material and which parts extend transversely to one another, the method comprising steps of: within a mould, placing a first constructional part and filling elements adjacent and extending transversely to a second constructional part, wherein the mould with the filling elements and the first and second constructional parts placed therein define a cavity, the cavity has a contour of a final product to be formed, and each of the constructional parts and the filling elements is a composite material comprising fibers and thermoplastic material; subjecting the mould, the filling elements, and the first and second constructional parts therein to heat and pressure sufficient to cause i) the thermoplastic material of the filling elements and the first and second constructional parts to melt completely and become fluid with the thermoplastic material flowing out, under pressure, into the cavity including a first connection region between the first and second constructional parts, and ii) fusing of the filling elements, the first constructional part, and the second constructional part in their entirety; and cooling the fused filling elements the first constructional part, and the second constructional part to obtain the final product having an outside contour matching the contour of the cavity of the mould. 